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Definition

A gamma-ray will interact with its medium in one of three different ways: photoelectric absorption, Compton scattering, and pair production. These different interactions change their probability of occurring depending on the energy of the gamma-ray and the atomic number of the material.

Photoelectric Absorption

In the photoelectric absorption, the incident photon disappears and a photoelectron is produced from one of the electron shells of the absorber. The kinetic energy that this electron carries off is Ee− = h − Eb, where Eb is the binding energy of the liberated electron in its original shell. This empty spot in the electron shell is quickly filled by electron rearrange-ment. This process causes the binding energy, Eb, to be liberated as well. This energy is liberated in the form of a characteristic X-ray or an Auger electron.

The photoelectric absorption interaction is the ideal interaction for gamma-ray spectroscopy. The photoelectron carries away most of the gamma-ray energy and then an X-ray or Auger electron carries away the remaining kinetic energy. Assuming an ideal detector, the sum of these energies will equal the energy of the original gamma-ray.

This is desired for gamma-ray spectroscopy because we are interested in knowing the energies of the various gamma-rays that are emitted by a source. In Figure 3.3, we see what the ideal photopeak created by mono-energetic gamma-rays of a single energy looks like.

Compton Scattering

The Compton scattering interaction is the scattering of a gamma-ray off of a free or unbound electron, thus creating a scattered gamma-ray photon and a recoil electron. The energy of the incoming photon is divided between the scattered photon and the recoil nucleus by a relationship that is dependent on the scattering angle. There are two extreme cases dictated by this equation: When  = 0, the scattered photon retains all of its energy and the recoil electron gains no energy. When  = , the incident gamma-ray is backscattered and the recoil electron moves along the direction of incidence. This case is the case with the maximum energy transfer between the incoming gamma-ray and the electron.

In the detector, all scattering angles from 0 to  will occur. Because of this, a continuum of energies can be transferred to the electron. This energy has a range from 0 all the way to the maximum.

Pair Production

Pair production is a gamma-ray that turns into an electron-positron pair. This occurs when the gamma-ray is in the intense electric field near the nuclei of the absorbing material. There is a minimum amount of gamma-ray energy that is required for this process to take place.

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